The present invention relates generally to the system utilized to recirculate electrolyte from a gas-liquid disengager to the electrochemical cell. More specifically, the present invention relates to an improved downcomer for an anolyte or catholyte return line that connects the appropriate gas-liquid disengager and each electrode within the chloralkali electrochemical cell in a manner which improves the efficiency of gas separation in the disengager and strengthens the cell structurally. The improved downcomer could be used equally well to return catholyte from the catholyte gas-liquid disengager to each cathode frame or to return anolyte from the anolyte gas-liquid disengager to each anode frame with the same advantages.
Chlorine and caustic, products of the electrolytic process, are basic chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chloralkali cells. The chloralkali cells today are generally of two principal types, the deposited asbestos diaphragm-type electrolytic cell or the flowing mercury cathode-type. Comparatively recent technological advances, such as the development of the dimensionally stable anode and various coating compositions, have permitted the gap between electrodes to be substantially decreased. This has dramatically increased the current efficiency in the operation of these energy-intensive units.
The development of a hydraulically impermeable membrane has promoted the advent of filter press membrane chloralkali cells which produce a relatively uncontaminated caustic product. This higher purity product obviates the need for caustic purification and concentration processing. The use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. However, advances continue to be made in the development of monopolar filter press membrane cells.
Replenishing the depleted electrolyte, typically a salt brine, has been accomplished in diaphragm cells by having feed lines carry a portion of the fresh electrolyte via external feed lines through external gas-liquid disengagers into a tank holding a plurality of electrodes. Those prior art structures which replenished the electrolyte internally either utilized the existing electrode frame side channels to carry the fresh electrolyte towards the bottom of the electrode or fed the electrolyte into the electrode from the top through short feed lines. The former method potentially weakened the frame structure or restricted the flow rate capacity to that achievable within the existing electrode frame design dimensions. The latter method failed to mix the fresh electrolyte thoroughly with the existing electrolyte in the electrode. Neither method optimized cell efficiency by ensuring maximum separation of the gases from the electrolyte prior to recycling and replenishing the electrolyte.
Monopolar filter press membrane cells are characterized by the utilization of hollow electrode leaves bordered by gasketed frames in which anode and cathode leaves alternate. Each anode and cathode is separated by an ion-selective permeable membrane, which is held between each pair of electrode frames. Because of the high cost of the materials required, it is desirable to have a maximum of electrode surface area per unit of cross-sectional area and per unit of volume for each electrode. However, this optimum in economy must be balanced by the practicality of design where it is necessary to make the electrode leaves thick enough to contain conductor bars, to permit the internal flow of gases and liquids, and to provide sufficient area for the attachment of inlet and outlet conduits. Compounding design problems is the proven fact that chlorine gas contact with the membranes utilized in the monopolar filter press membrane electrolytic cells tends to accelerate deterioration of the membrane structure, thereby negatively affecting the performance and life of the membrane. External anolyte and catholyte gas-liquid disengagers have been employed recently in an attempt to maximize the available electrode surface area per unit volume for each electrode. Thus, it is advantageous to separate as much as possible of the chlorine gas from the anolyte outside of the anode; in other words, in the anolyte gas-liquid disengager so that there is minimal exposure of the membrane to the chlorine gas.
Similarly, since hydrogen gas is produced in the cathode during electrolysis, it is desirable to have as much of the hydrogen gas as possible removed from the catholyte in the catholyte gas-liquid disengager to improve the efficiency of the cathode while structurally strengthening the cathode.
The foregoing problems are solved in the design of the apparatus comprising the present invention by providing in a filter press membrane electrolytic cell external gas-liquid disengagers to maximize the ratio of the electrode surface per unit of cross-sectional area and per unit of volume so as to separate out entrained chlorine gas from the anolyte fluid and hydrogen gas from the catholyte fluid at desired rates by providing a downcomer or anolyte return line for each anode and a downcomer or catholyte return line for each cathode that have a first portion external to each electrode and a second portion internal to each electrode, the first portion being at least partially generally circular in cross-section and the second portion having in cross-section a generally arcuate periphery with a predetermined cross-sectional dimension such that the downcomer is contiguous to the opposing sides of the electrode in a structurally reinforcing manner so that the structural rigidity of the electrode is increased while the electrode surface area available for fluid contact is maximized per unit of electrode cross-sectional area and per unit of volume while permitting electrolyte to be circulated from the disengager into the electrode during the electrolytic process.